Here is code that works:
Mail::to($emails)->send(new ExceptionOccurred($e));
And then I change it to:
Mail::to($emails)->queue(new ExceptionOccurred($e));
When I do I get the error:
ErrorException: Undefined property: App\Mail\ExceptionOccurred::$content in C:\inetpub\wwwroot\laravel\app\Mail\ExceptionOccurred.php:33
This is ExceptionOccurred.php:
namespace App\Mail;
use Illuminate\Bus\Queueable;
// use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;
class ExceptionOccurred extends Mailable
{
    use Queueable, SerializesModels;
    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct($content)
    {
        $this->content = $content;
    }
    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->markdown('mail.ExceptionOccurred')
            ->subject('Exception on live instance')
            ->with('content', $this->content);
    }
}
This is the relevant portion of the exception handler:
if ( $exception instanceof Exception ) {
    $e = FlattenException::create($exception);
} else {
    $e = $exception;
}
$emails = json_decode( env('MAINTAINER_EMAILS') );
if (app()->environment('production') || app()->environment('testing') ) {
    Mail::to($emails)->send(new ExceptionOccurred($e));
}
To re-iterate, Mail::send() works, but Mail::queue() does not. I believe the queue is set up correctly.
 
    